Even though Emily said Madonna hasn't been booked it almost seems like it makes too much sense. She's putting out new material in the spring and with the lack of a legacy headliner this year, she'd go a long way in filling that spot.
Madonna may not have been booked when they thought they had Macca, but obviously things have changed. Also, for those of you still convinced Gaga is a possibility, I highly doubt she does any shows in 2019 outside of Vegas. When Britney had her residency (same promoter/venue) she was not allowed to do any shows outside of Vegas for the first 2 years, due to a clause in her contract. With the demand for the Gaga shows, and the rumor that they're added 25-30 new dates I highly doubt they'll let her perform anywhere else.

They played 3 sets at Roo back in '09 and 2 sets in '13. They're one of the largest touring acts in America. Also, having been to Bonnaroo eight times, the way that they do their headliner sets is interesting because they're all unopposed. During the headliner no other stages have music on them. It allows the whole festival to gather at the main stage and take in an act, whether they're fans or not, because there's nothing else going on.

Over on Inforoo (US version of eFests) they're saying that Arctic Monkeys have pulled out of their US tour dates for June (MSG and Bonnaroo). Could they be pulling out in anticipation of playing some Euro shows along with Glasto?

Firefly Festival in US was announced yesterday, it's the week before Glasto, usually very little carryover. Anyway, names of note that are on the lineup include Travis Scott, Post Malone, Courtney Barnett, Brockhampton, Vampire Weekend, Lykke Li, Death Cab for Cutie, Young The Giant, and Tyler The Creator.